For audio coding, the MPEG-2 standard provides for a spatial representation by multi-channel reproduction (multi-channel sound). The multi-channel sound in this case comprises three channels L, C, R (Left, Centre, Right), which are arranged spatially in front of the listener, and two channels LS, RS (Left Surround, Right Surround), which are arranged spatially behind the listener. In addition, a sixth channel for special effects is provided. In order to ensure backwards compatibility with MPEG-1 signals, the signals of the multi-channel sound channels are matrixed. The stereo signals L0 and R0 calculated in the process are then transmitted as MPEG-1-compatible stereo signal and the remaining three audio signals are transmitted as supplementary data.
Before being matrixed, the audio signals generally have their levels reduced in order to prevent overdriving. Thus, a value of L0=2.4 would result, for example, in the case of matrixing in accordance with L0=L+0.7C+0.7LS without attenuation, i.e. for L, C, LS=1. In order to balance again the reduction at the encoder end, an option that is provided in the MPEG-2 standard is for a decoder to raise the output levels again. If, however, a transmission error then occurs in the multi-channel section, the decoder cannot carry out the reverse matrixing. In that case, only the MPEG-1-compatible signal component, i.e L0 and R0, will be decoded. The auditory impression can therefore be disturbed in various ways during the changeover to MPEG-1 decoding. Thus, sudden changes in volume may occur, for example, since level raising is not carried out in MPEG-1 decoding.